The blades of a rotating axial impulse turbine wheel create a pressure gradient causing what is effectively a pumping loss. This results in a loss in power and, hence, a decrease in the efficiency of the turbine.
The magnitude of the pumping loss depends at least in part on the axial clearance between the rotating turbine blades and the stationary turbine wheel housing. To this extent, the smaller the clearance, the smaller the pumping loss. Turbine design practice commonly provides for close axial clearance between the blades and the housing on the inlet side of the turbine wheel, with the outlet side usually left open to an exhaust port or duct.
Some turbines do not have nozzles covering an entire three hundred and sixty degree annular area available at the turbine blade inlet face and are commonly known as "partial admission" turbines. Since only the part of the blades passing the nozzles are filled with "driving" gas, the remaining or "unadmitted" part of the blades consume power via the aforementioned pumping loss.
The object of the present invention is to dispose a cover on the downstream side of the turbine wheel rotating assembly to reduce this pumping loss.
The present inventor is aware of the following prior art relating generally to turbines: U.S. Pat. No. 3,139,264 for a Canted Vortex Venturi which issued to Quennville, et al on Jun. 30, 1964 (U.S. Class 253-65); U.S. Pat. No. 2,608,807 for a Precision Hole Grinder which issued to Nilsen, et al on Sep. 2, 1952 (U.S. Class 51-245); and U.S. Pat. No. 4,684,321 for a Heat Recovery System Including A Dual Pressure Turbine which issued to Barrett, et al on Aug. 4, 1987 (U.S. Class 415-202).
The '264 patent teaches a casing which appears to cover the rear face of a turbine. The function of the casing is to insure that all of the turbine wheel exhaust flow is forced to exit through a canted vortex venturi and to maintain the back pressure created by the canted vortex venturi. The present invention, on the other hand, increases turbine efficiency by minimizing the pumping loss of the blades outside of the nozzle periphery and thus relates to a different structural arrangement.
The '807 patent teaches a casing downstream of a turbine wheel and which casing does not extend far enough radially to cover the turbine blades. This is contrary to the present invention which covers the downstream side of the turbine blades to minimize pumping loss.
The '321 patent teaches a casing downstream of a first stage wheel for providing an inlet passage for a secondary flow entering the nozzles of a second stage. The structural arrangement of the '321 patent does not relate to reducing pumping loss with a downstream turbine blade cover as does the arrangement of the present invention.